1. Field
Embodiments described in the present specification relate to an electrically rewritable nonvolatile semiconductor memory device.
2. Description of the Related Art
In a nonvolatile semiconductor memory device having a charge storage layer which is a floating gate or some other SONOS type or MONOS type charge storage layer, data is stored by retaining charges (electrons or holes) in the charge storage layer. The threshold voltage of the memory cell transistor changes due to the quantity of retained charges, hence data can be determined by applying an intermediate voltage around that at which the threshold voltage changes as a gate voltage of the memory transistor and seeing whether the memory transistor becomes conductive or not. In this kind of nonvolatile semiconductor memory device, data write and erase is performed by a tunnel current from the semiconductor substrate or control gate due to a high electric field, or hot carriers generated from the semiconductor substrate.
Incidentally, when a write and erase operation is performed repeatedly on one memory cell, an insulating film of the memory cell suffers damage due to the charges passing through it, and is thereby gradually degraded. Electron traps and hole traps are generated within a degraded insulating film and charges are retained in these electron traps and hole traps similarly to in the charge storage layer in a floating gate electrode or the like. As a result, during information (data) retention, the memory cell retains charges in both the charge storage layer and the traps within the insulating film. This charges retained in the traps differs from the charges retained in the charge storage layer in not being assumed to be retained after the write operation. As a result, the charges retained in the traps exert a large effect on write characteristics of the cell.
As miniaturization of memory cells advances, the amount of charges required to cause threshold voltage change in the memory cell is reduced. That is, along with miniaturization, threshold voltage changes resulting from whether one electron trap is trapping electron or not also become significant. Particularly in multi-value memory where two bits or more of information is stored in one memory cell, threshold voltage is required to be controlled within a narrow threshold voltage distribution range, resulting in the need to suppress effects due to charges retained in the traps.